There is increasing demand for individual wireless communication devices that are operable with more than one communication system, each having a different Radio Access Technology (RAT). This may be desirable in applications where the alternative systems available have different features and can provide different services. For example, in order to deliver high performance, low-cost mobile voice and data services to mobile subscribers at home and/or the office, in addition to the typical mobile services, dual mode communication devices have been developed that are operable on a cellular communication system, such as a second generation (2G) system like the Global System for Mobile communication (GSM), or a third generation system, like the Universal Mobile Telecommunication System (UMTS) or other systems based on Wideband Code Division Multiple Access (WCDMA) technology, and an Unlicensed Mobile Access (UMA) communication system such as a Wi-Fi system. UMA or Global Access Netork (GAN) is a Fixed/Mobile Convergence standard (FMC) part of a 3rd Generation Partnership Project (3GPP) for subscriber access to mobile circuit, packet and IMS-based services over any IP based access network, including the internet. In another example, it may be desirable for cellular communication devices to be operable with two or more of 2G, 3G and 4G communication systems.
Typically, multi-mode devices that are capable of operating with multiple communication systems will have a preferred communication system out of the multiple communication systems. The preferred communication system is determined according parameters such as what services are available on the communication systems, cost of services, Quality of Service (QoS), network identification and potentially user preference. If the quality of communication provided by the preferred communication system is poor and below a predetermined threshold, then a handover or reselection will be triggered and the communication device will switch to another communication system with better quality.
The wireless communication device may operate in one of several modes. In an idle mode, the communication device is registered with a communication system, which will normally be its preferred communication system unless the quality of communication is poor, and will receive signalling information, such as paging messages on a paging channel, from the communication system but there will be no Circuit Switch (CS) or Packet Switch (PS) connection for communicating traffic information such as in a voice or data call. In an active mode, communication resources will be allocated to the wireless communication device and there will be a Circuit Switch (CS) or Packet Switch (PS) connection for a voice or data call.
With battery powered communication devices, there is always a need to reduce power consumption in the devices so as to increase battery life. As is well known, in 2G and 3G systems, in order to improve battery life, when the communication device is not in an active mode of operation (i.e. not transmitting or receiving traffic information, such as SMS, video, multimedia or voice data in a voice or data call), the wireless communication device enters an idle mode of operation in which some of the components of the communication device, such as the RF reception block, are powered off to save power. This is known as the discontinuous reception (DRX) mode. In the DRX mode, the components of the communication device that are required to monitor the signalling signals from the communication system(s) will still consume power but this is small compared to the consumption in the active mode.
Other techniques have been proposed for limiting power consumption in the idle mode. For example, U.S. Pat. No. 7,133,702 and PCT application no. WO 2004/021731 describe techniques for limiting cell reselection in idle mode so as to reduce power consumption. PCT application no. WO 2007/035737 describes a technique for quick detection of signalling when decoding a paging channel in order to reduce power consumption in the idle mode.
For a multi-mode communication device operating in the idle mode, the power consumption in the device can be greater on some communication systems than on others. For example, for a communication device in idle mode, the power consumption in the device is greater when the device is operating with a Wi-Fi communication system than it is when the device is operating with a GSM communication system since the Wi-Fi system does not use a DRX mode which is as efficient in saving power as the DRX mode used on GSM or UMTS. For example, the current drain (which is equivalent to power consumption) of a wireless communication device operating in idle mode on a Wi-Fi communication system is between 6-10 mA whereas the current drain of a wireless communication device operating in idle mode on GSM communication system is between 1-3 mA depending on the DRX mode.
Thus, the power consumption of a multi-mode communication device operating in the idle mode will depend on what communication system has been selected to be its preferred communication system.
For a multi-mode communication device operating in an active mode, the differences in power consumption between the different communication systems are less significant and depend more on the quality of the link between the communication device and the communication system.
In order to increase battery life in a multi-mode communication device, it would be desirable to reduce power consumption in the communication device when it is operating in idle mode.